Why the 270 Degree “Crossplane Twin” is Suddenly So Popular

Summary

This retrospective examines the evolution of twin-cylinder motorcycle engines over the past century, explaining how the predominant designs shifted from 360 to 180 to 270-degree crankshafts. It delves into the engineering considerations of vibration, power, emissions regulations, and manufacturing that influenced these changes. The narrator argues 270-degree 'crossplane' twins are a compromise, as the theoretically optimal angle is actually 285 degrees, which only KTM builds despite higher costs. Overall, an in-depth analysis revealing it took 100 years to fully optimize twin motorcycle engines, though they still aren't quite perfect.

Takeaways

• 😀 Early twins were 360° for smooth power delivery from imprecise carburetors
• 😮 180° twins enabled higher RPMs and power from lighter crankshafts
• 🤔 270° twins balance vibration for bigger, faster pistons
• 😠 Emissions regulations drove adoption of smoother, cleaner twins
• 😡 Total crevice length causes more unburnt hydrocarbons in 4-cylinders
• 😱 Secondary vibration worsens with bigger, faster twins
• 🧐 270° 'crossplane' twins offset this vibration through crank timing
• 🤨 Perfect crossplane is really 285° for secondary force balancing
• 😒 Manufacturing costs have limited adoption of true 285° twins
• 😫 KTM builds proper 285° twins despite higher machine costs

Q & A

Why were early twin-cylinder engines made as 360° twins?

Early twin-cylinder engines were made as 360° twins because that configuration allowed the use of a single carburetor to supply fuel evenly. The paired pistons moving together provided balanced power delivery that worked well with less precise carburetors.

What are the advantages of a 180° twin cylinder layout?

A 180° twin has a lighter crankshaft that can rev higher than a 360° twin. The lighter reciprocating mass allows higher rpm operation. Also, the constant crankcase volume improves efficiency.

Why have manufacturers recently switched to 270° twins?

To meet tighter emissions regulations, manufacturers have switched to 270° twins to reduce unburned hydrocarbons. The 270° layout balances secondary vibrations that become more significant in larger, higher performance twins.

Who first proposed the 270° twin cylinder layout?

Phil Irving first proposed an offset twin cylinder firing order in 1962, but not specifically 270°. He envisioned a 285° firing order to balance secondary vibrations.

Why wasn't the 270° twin adopted earlier?

The manufacturing process for an offset crankshaft was more expensive. It requires an additional twisting operation compared to an inline crankshaft.

What company first produced a 270° twin?

In the 1990s, Dick Cookson built the first 270° twin by modifying a Triumph Bonneville using parts from a nuclear submarine.

When did manufacturers begin adopting 270° twins?

Manufacturers only recently began adopting 270° twins, with Honda releasing their Africa Twin in 2016. Improved and affordable machining made it practical.

What is the optimal offset angle for a crossplane twin?

The optimal angle is actually 285°. This balances secondary vibrations while keeping maximum piston velocity centered between TDC and BDC.

Why are manufacturers using 270° over 285° twins?

The 285° manufacturing process remains more expensive. 270° crankshafts can be forged without an additional twisting operation to create the offset.

Who is producing 285° twins?

Currently, KTM is the only manufacturer producing 285° twins. They have invested in the more complex manufacturing to optimize performance.